Assuming you send a message from a function node, to a knx-ultimate node with Group Address and Datapoint properties set.

To DIM a lamp or other devices, such as blind for example, you need to pass the direction of the dimming (increase/decrease) and the value of the dimming (the realative %)
Create a knx-ultimate node with the Group Address to be dimmed, with the Datapooint set to 3.007.
Please note that you need to uncheck "Send payload to KNX only if changed (RBE filter)" in the "OUTPUT (send datagram to the KNX BUS)" section of the knx-ultimate config advanced options. Then add a function node, and paste this code

Increase the light/open blind

// The parameter "data" indicates the relative amount of the dimming commmand (how much to dim).
// The parameter "data" can be any integer value from 0 to 7
// The parameter decr_incr:1 increases the light
// The parameter decr_incr:0 decreases the light
msg.payload={decr_incr: 1, data: 5};
return msg;

Decrease the light/close blind

// The parameter "data" indicates the relative amount of the dimming commmand (how much to dim).
// The parameter "data" can be any integer value from 0 to 7
// The parameter decr_incr:1 increases the light
// The parameter decr_incr:0 decreases the light
msg.payload={decr_incr: 0, data: 5};
return msg;

To set the absolute brightness value of a lamp or other devices, such as blind for example, you need to the absolute brightness value to the knx-ultimate node
Create a knx-ultimate node with the Group Address and the Datapooint set to 5.001.
Then add a function node, and paste this code

Set brightness of the light/set the position of the blind

msg.payload=50; // values from 0 to 100
return msg;